There is this idea that we find in a few books, mainly The Swarm by Frank Schätzing and in the Ender Saga by Orson Scott Card, that some species could communicate via DNA exchange.

Technically, yes DNA is information and with the rise of computers, we're all well aware that binary is enough to encode most information (DNA is quaternary). Also, it is known that some organisms (mainly viruses (yes I know viruses aren't technically organisms)) are able to inject genomic material into a host.

So there you are. It seems technically plausible.

But how realistic is it?

How fast would it be? Faster than social communication? Faster than hormonal diffusion? Faster than neuronal propagation?

What would be the advantages and disadvantages of such organisms? What would be the particular traits implied by using such way of communication? (I'm thinking: do they have to live in water? do they have to be single-cell? ...)

$\begingroup$Well when DNA is "exchanged" during sex, the information contained in it isn't acquired by any of the participants. I mean a biology where what you "know and think" is contained in your DNA the way it is in our brains, and for talking you inject DNA in your partner's genome.$\endgroup$
– SheraffOct 8 '14 at 22:08

$\begingroup$I know, but under the strict definition of communication (transfer of information) it applies.$\endgroup$
– TelastynOct 8 '14 at 22:12

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$\begingroup$Would this imply that beings are capable of altering their DNA to communicate to one another, or DNA is constant and one being is sending out the same message for his/her lifetime?$\endgroup$
– TwelfthOct 8 '14 at 22:32

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$\begingroup$That sort of information transfer is also common between bacteria.$\endgroup$
– celtschkOct 8 '14 at 23:08

4 Answers
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There are both advantages and disadvantages to this proposed method of communication.

First the advantages:

You could transport massive amounts of data, gigabytes and gigabytes with a single transfer. That data could also be copied by the target and passed on in a similar way.

You could be sure that communication is secure, no-one is able to listen in without physically getting a sample of the transfer.

But now the disadvantages:

Range - you need to be in touching distance, no shouting (unless you released spores into the wind and hoped someone collects them).

Speed - it would take time to synthesize the DNA. In the plant example DonyorM mentions the parasite is always injecting the same mRNA into the host, it doesn't have to compose dynamic messages. It might also take time to read the DNA, certainly it would be unlikely to be instantaneous.

Reliability - DNA copying can introduces errors. Depending on how many times the information was being copied you may need error checking and recovery mechanisms in place. Memory is already notoriously unreliable though so this would be manageable.

Considering most communication systems evolved from early warning where rapid spread and reaction was important (for example warning cries that a predator is approaching) then DNA would not be a suitable mechanism to do so.

Something like an ant colony would be more likely to evolve along these lines though, as they already use chemical signaling and close-contact communication. For example the queen might compose orders to the workers in the form of DNA strands and then hand them out to be acted on. Scouts might synthesize the report on their way back to the base and then use them to report, etc.

$\begingroup$Cool answer :-) even though I don't really agree with reliability. The amount of error during DNA operations is negligible in face of social ambiguity. So maybe it is a disadvantage compared to a perfect communication system, but not compared to ours.$\endgroup$
– SheraffOct 9 '14 at 8:58

$\begingroup$Based on your last paragraph, I'd then assume that for a population to develop this trait with an evolutionary gain, it would have to be a big group in close proximity at all times. Wouldn't that be either very cute or very scary?$\endgroup$
– SheraffOct 9 '14 at 8:59

$\begingroup$@Sheraff I agree, it would be no less reliable than memory (for example). I'll add a caveat.$\endgroup$
– Tim BOct 9 '14 at 9:02

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$\begingroup$As for the unreliability, a good language has a fair bit of redundancy, so that if data gets corrupted, the meaning is still easily deduced.$\endgroup$
– overactorOct 9 '14 at 10:18

Plants do this, in a way. When a parasitic plant attaches to its host plant, the parasitic plant sends lots of mRNA (messenger RNA) into the host plant. This mRNA then allows the "hijacking" of the host plant. RNA controls what proteins are made in cells, so invasive mRNA could cause the plant to lower defense, move resources to the parasitic plant, etc.

I think this kind of communication would certainly be possible among sentient organisms. Probably the organisms would have special "ports" where the mRNA is put into the listening organism. Then inside the port, special cells take over the mRNA and create specific "language" proteins. Then the brain or receptors read these proteins and construct a word/sentence with them.

This method of communication would be as fast as the proteins and mRNA could be produced and transferred. Likely this would wouldn't be too slow. It would require active transport, but would be faster than any kind of diffusion based communication. But it would be slower than both talking and neural propagation. The difference may be minimal, it depends on what biological functions developed.

There are a few advantages to this system. First, you always know someone is listening. They can't not listen. Second, only the people you want to hear you can hear you. It requires a physical tether to hear someone talk. This is also a disadvantage, it prevents long-distance communication. This can be gotten around with technology. Technology could save the patterns of the mRNA or proteins on one side of a "phone line," and then on the other side they could be reconstructed and read. Another disadvantage is that it takes more materials. For all inter-individual communication the organism has to build proteins, which takes energy and food.

There would be no special parameters to how this organism would live/how it looked. But it would have to have the special "port" for receiving mRNA. Different environments would have this port look different (underwater it would need to be waterproof, for example).

I've read science fiction stories where memories and/or knowledge is inherited through RNA transfer from mother to child. It seems completely plausible that an organism might evolve that encodes data in DNA, and "spits" it at others of its same type to transfer large amounts of information. Probably not useful for immediate danger scenarios, but there's no reason why the organism couldn't have a second communication method for quick, low-information transfer. (i.e. "Hey, look out!")

The real question of plausibility has to do with whether anything like that would evolve in the first place. Specialized functions pretty much always evolve from more generalized body parts, and they only evolve as much as they have to to provide an advantage. Humans don't have hands to use tools -- we have hands because our genetic ancestors climbed trees as a way to stay safe and gain access to more food sources, and then we evolved the ability to use our brains in conjunction with these proto-hands to be able to use simple tools. Using tools ended up being so advantageous that some of our ancestors out-competed their peers by having hands that were slightly better at using tools, or by being able to use their brains better to think up new ways to use tools, or both.

Now apply the same idea to communication systems. Effective communication is, obviously, a major evolutionary advantage -- not just in humans, but in all social animals, be they pack hunters like wolves, or colony insects like bees and ants. Communication systems evolve in many different ways, piggybacking on existing organic traits, and using transmission media that is readily available, and low energy. Sonic communication is easy in our atmosphere, and also underwater -- sound travels readily, and is simple to produce mechanically. There's a reason why there are so few organisms that communicate via actively produced light. But using existing light to communicate happens all the time. Humans and other mammals use "body language", which is just light reflecting off an organism in a particular pattern. Bees do it too, with their "dances" to communicate the location of and directions to flowers. Of course, visual communication doesn't work in the dark.

Another widely-evolved communication system is chemical. Pheromones and scent glands communicate with a high degree of nuance, and ants produce pheromones to communicate up to twenty different ideas. DNA is, of course, just a very complex chemical, one that gets synthesized in large(ish) quantities already as part of the reproductive system of all sexual organisms. Thus, a DNA-based communication system for advanced, intelligent organisms would likely piggyback on their existing reproductive organs.

Of course, there's also the possibility that these DNA-communicating intelligent lifeforms altered themselves countless generations ago to provide for this DNA communication system, but their society has since regressed to the point where the origin is effectively prehistoric.

There are a bunch of good answers here that are clearly showing that information beyond genetics can be encoded in DNA and those are indeed answers to the question as it is phrased. But the question made me wonder about DNA, not as written language, which is implied by the asker, but rather as "spoken" language - I doubt that any of us would find the coding discussed so far a suitable and satisfactory substitute for what we call language in our daily lives. So I think the we need to answer (at least I want to answer), "can DNA be used by a species just the same way we use spoken language or sign language". Dynamic, interactive, adaptive symbolic code capable of communicating, on a contemporaneous basis, information, emotion, etc.

Can DNA have a syntax? A grammar? Yes, we know genetic coding already has both syntax and grammar so those tools exist for our DNA language.

We need a organ to "speak" our language and an organ to "hear" our language. We can certainly imagine such organs. Our cellular machinery can already perform the tasks of writing (copying) and reading DNA strands. But our imagined language organs will need to be much more nimble. Our speech organ will need to assemble DNA strands based on inputs from our nervous system; Building DNA strands representing words and sentences and then emitting them to the medium between the speaker and the listener (air, water, ??). Such an organ would probably need to emit many copies of our sentence since our words are traveling in matter packets (not energy waves) and cannot be "heard" until the molecule of our sentence arrives at the receivers hearing organ. Sending just a single sentence molecule would result in a very poor chance of ever being heard! We must puff our sentence out a thousand times simultaneously like pollen from a flower on the wind. Of coarse, our words and sentences will need to contain a grammar similar to a time stamp. If I say "Hi, How are You?" 1000 times, and the recipient receives 120 of those "Hi, How are You's?", I don't want him to "hear" all of them. I need to give him a grammar to hear the first "Hi!" and filter out all the rest as noise.

So, on the "hearing" end of the conversation, we need an organ filtering the media between us and the "speaker" scanning for floating bits of DNA. Assuming we are on a planet that uses DNA for genetics, not just language, there is going to be a lot of DNA genetic material floating about. So our DNA language grammar must make a clear distinction between a word or sentence and genetic DNA which is just noise to be filtered out and ignored. If this filtering happens at the biological level of the organ itself, then calling it a "grammar" may be misleading. For instance, the organ might only bond with DNA strands that begin with a complex 100 nucleotide sequence. Any DNA strand that does not have this telomere at the beginning/end does not bind and is not "heard". Either way, biological or grammatical, the hearing organ responds to the coding in the captured and read DNA strand by activating the hearer's nervous system. Just as our ears do for us.

So now that we have imagined the machinery for a species to have DNA molecule based language, we can imagine some consequences and advantages...let's go scifi.

In the case where we use the biological filters on our organs (complex telomere tag is required or molecule is not read because it wont bind) any individual who suffered a mutation that either resulted in there speaking organ coding the wrong telomere or there hearing organ failing to bond to the correct telomere would be rendered either "mute" or "deaf".

Conversations would be subject to very different rules when your words can be blown with the wind and be heard miles away! Or when no one up wind of you can hear you no matter how loud you shout. Also, since your words and sentences are subject to the vagaries of the wind or just random distribution by Brownian motion, our language and our nervous system will need to evolve to deal with the fact that our sentences will often not arrive with the same timing, or even in the same order, that we spoke them in!

And for that matter, what is a shout? Producing 100,000 DNA sentence copies at a time? Making a whisper the creation of just a few copies?

As this species evolves, will they evolve some level of encoding to enable private conversation? Perhaps the hearing organ can hear a DNA molecule starting with any telomere sequence, but it can hear (bind) only one possible telomere at a time. So most conversations happen with telomere Alpha. But when two or more individuals want to talk voce sotto, they switch over to telomere Lambda or Omicron. There words are still arriving at the hearing organs of others, but are ignored by all who are not "listening" for Omicron.

What is a recording device on this planet? Can biological microbes be engineered to reproduce / record conversations by replicating the DNA that makes up the sentences and words?

Since words are matter, not energy, they potentially have a lifetime far beyond what we are used to. You could capture the last words of your mother in a jar, and hear them again...and again....and again for years - until the last of the molecules of that sentence were used or lost. NOT just a recording - the actual voice of your mother, her spoken words from her "mouth", unchanged just as if she was there with you in the room.

Specters of conversations might linger in buildings for years or generations. Imagine closing a door in Philadelphia's Liberty Hall and dislodging from a dusty rafter a few sentences between Ben Franklin and Thomas Jefferson arguing over a paragraph in the Declaration of Independence! As those sentences/molecules drifted into your "ear", you would hear Franklin and Jefferson - their words in their voices - unchanged from the moment they spoke them 250 year ago. The occupations of archaeologist and historian would be something quite different.

Finally, since DNA is still the mechanism for genetic information encoding and transmission, can these creatures "speak" genetic information? For instance, could a lover "whisper" a hormone that caused the object of there affection to feel flush? Or an RNA strand, inhaled by the subject which caused the production of the hormone by his/her own limbic system. Or could a rival encode and surreptitiously breath a virus onto his victim? Or, the most insidious end to this line of reasoning...could a sentence itself be encoded in such a way that hidden within it were the very instructions that caused the hearer's cells to produce the virus in their own body?! Just like sticks and stones, on such a world, words can kill you!